When producing or processing steel cords at one point the cord comes to an end. There is therefore the need to connect ends of steel cords to one another to continue processing. The connection should be easy to make and have properties close or equal to that of the steel cord so the connection passes unnoticed in the process.
The current practice is to connect steel cord ends by means of welding. The ends of a steel cord are cut flush and mounted in the moveable clamps of a welding apparatus. While the ends of the steel cord are pressed against one another alternating electrical current is applied through the clamps and the cord ends heat up to the point that the steel softens and melts. The ends are pushed into one another over a set distance and as soon as this distance is reached the current is cut off. The weld cools down quickly in the ambient air. This procedure is known as ‘upset welding’ or ‘resistance butt welding’.
Steel cords are made of fine (thinner than 0.50 mm) plain carbon steel filaments that are cold drawn to high tensile strength (higher than 3000 N/mm2). Two or more filaments are twisted together into a steel cord. The metallographic structure of the filaments is a high tensile, drawn pearlitic steel structure. In the weld the strength increase due to cold forming is completely lost. Due to the high cooling rate after making the weld the metallographic structure becomes martensitic which is more brittle than the drawn pearlitic structure. In the current welding procedure it is not possible to prevent the formation of the martensitic structure as the cooling of the weld is too fast, and in any case much faster than when compared to the welding of thicker wires.
In order to alleviate the brittleness of the martensitic steel in the weld, an annealing operation—also called stress-releaving thermal process—is subsequently performed by placing the weld between two annealing clamps. By driving a small alternating current through the weld, the weld is annealed. An annealed martensitic structure has more ductility. GB1251928 describes such procedure.
As extra material is added into the weld zone, the transverse size of the weld is larger than that of the steel cord and the burr must be buffed or hammered away so that the diameter of the steel cord does not appreciably increase at the weld. In general a diameter increase of not more than 3% is allowed.
The procedure is difficult to repeat in a consistent way because:                An AC current source is used wherein a high line voltage of 380 to 400 VAC is down converted to a low voltage, high current by means of a transformer. Fluctuations in line voltage directly influence the power emitted at the weld. These fluctuations can be significant over time depending on the quality of the electrical grid in the wire plant.        The steel cords have an irregular surface with which it is difficult to make a consistent and repeatable electrical contact. The problem is particularly severe when welding open cord type of constructions or constructions with few filaments such as 2×1 wherein steel filaments can shift over one another and are difficult to align face-to-face in the clamps. The electrical resistance therefore can differ very much from clamp event to clamp event even on the same steel cord. When welding single wires, this problem is not an issue as the smooth surface of the wire provides a consistent and repeatable contact and the end-faces of the wire can be easily aligned.        
The procedure results in a weld that has less than desirable properties: the inferior ductility of the annealed weld, the reduced strength of the weld and the presence of a weld burr make such a weld a weak spot. Welds are therefore subjected to a tensile test, a diameter check and sometimes a ductility verification. If the weld fails it has to be redone which is a loss of time.
In the existing art mostly reference is made to the welding of single wires or filaments of relatively thick gauge (more than 0.5 mm) such as wires for welded mesh.
U.S. Pat. No. 3,818,173 describes a method to weld lead patented (i.e. the wire has not been cold drawn), high carbon (more than 0.45 wt % C) steel wires end-to-end wherein first an annealed martensitic weld is made. In a following, separate step the weld is subjected to a ‘homogenising step’ above a critical temperature (1065° C., i.e. above austinisation temperature) for several minutes and subsequently is ‘cooled under a controlled cooling rate’. It is suggested to ‘direct a blast of cooling fluid on the joints so as to cool the steel rapidly to a subcritical temperature at which the carbon in the steel transforms directly to pearlite in minimal time’. During this step the temperature is measured by a pyrometer of which the temperature reading is used to control the current through the weld. The procedure comprises several steps, is therefore lengthy (takes several minutes) and is not practical as a pyrometer has to be trained on the wire itself. Also it relates to single wires and not to steel cords.
CN 102328148 describes method for welding mesh (i.e. cross-wise welding) of steel wires of low or medium carbon steel wires wherein, after the formation of the weld, a first natural cooling step is followed by a separate heating step. After a second natural cooling step a final tempering treatment is applied. The method is only applicable for cross-wise welding of single wires in a mesh. In that configuration contact resistance between clamps and wire is very well repeatable.
CN101596643A describes a method to butt weld stainless steel wires. The method describes how the temperature can be controlled by means of a DC current pulse train. Only the magnitude of current and length of pulse is controlled.
DE2658332 describes a method to flash weld filaments or strands whereby the welding clamps are provided with recesses to receive the weld globule. In flash welding an electric arc is drawn between the filament or strand ends to heat those ends up. Flash welding is not used on steel cords as the ends are too irregular to ensure a repeatable arc formation.
WO 2008/116469 describes a welding method and the accompanying welding apparatus for welding duplex steel stainless steel strips wherein the cooling of the welded interface section is controlled by means of computer programmed to apply current during at least a part of the cooling cycle.
The following art has been identified that particularly focusses on the welding of steel cords:
WO 03/100164 describes in FIG. 5, page 9 and 10 the known procedure of welding and annealing a strand in a multi strand steel cord, further comprising the step of shortening the lay length of the strand prior to welding. Only one welded strand is present at a specific spot of the multi strand cord.
WO 2007/020148 describes a connection of steel cord ends that is particularly suitable to connect open cord type of steel cords. The connection comprises a jointed section such as for example a weld and a fixation section, away from the jointed section for immobilising the filaments relative to one another.
While the latter two procedures result in acceptable welds for their specific purpose they are not universally applicable or need extra materials and work.
The inventors therefore sought to improve the existing types of welds, weld procedures and welding apparatus in the manner that will be explained hereinafter.